Many processes are known for applying hydrous oxides of metals such as silicon and aluminum to titanium dioxide pigments to produce durable pigments having varying levels of physical and optical properties. In the main, these processes have consisted of batch-type processes which, in order to produce a pigment of high durability, require long treatment times. For example, U.S. Pat. No. RE 27,818 discloses a batch process for preparing titanium dioxide pigments having deposited thereon a dense inner coating of amorphous silica and an outer coating of alumina which pigment is reported to exhibit outstanding durability, high opacity and good dispersibility. To achieve these properties, control of the temperature, time and pH of the reaction medium during silica deposition is essential. However, the process described in this reference is time consuming, typically requiring long acid addition times (e.g., over four hours for an 8 percent silica coating) to slowly lower the pH of the reaction medium to precipitate the silica.
Further examples of batch-type processes are the processes taught in U.S. Pat. Nos. 3,928,057; 3,954,496 and 4,075,031. All of these references disclose processes in which a dense silica coating is applied to a titanium dioxide pigment by slowly lowering the pH of the reaction medium to precipitate the silica.
In addition to the above mentioned batch processes, a number of continuous processes for preparing durable titanium dioxide pigments having silica and alumina deposited thereon also are known. Purportedly these processes are capable of overcoming the shortcomings of the batch-type processes, namely the excessively long processing times. One such process is disclosed in U.S. Pat. No. 4,125,412. In this patent, a process is described wherein a slurry of titanium dioxide is prepared and heated to a temperature of from 80.degree. C. to 100.degree. C. To this heated slurry is rapidly added a solution of sodium silicate. Upon completion of this addition, the pH of the slurry is adjusted to from 9 to 10.5 and maintained at this value for from 15 to 60 minutes to cure the silica coating. The pH of the slurry then is adjusted to about 8 and rapid addition of sodium aluminate and acid simultaneously are commenced in a manner sufficient to maintain the slurry pH at a value of from 7 to 9. Finally, the slurry is filtered and the recovered pigment washed and dried.
Another known process is the process described in European Patent Application No. 82/106706.9 published Mar. 9, 1983. According to this published application a slurry of titanium dioxide pigment, having a pH of from 9.8 to 10.1, is heated to at least 85.degree. C. To this slurry is added a soluble silicate compound in an amount sufficient to provide from 0.6 percent to 10 percent of silica as a dense silica coating. The slurry then is neutralized in three separate steps or stages by the addition of sufficient amounts of an acid to achieve pH levels of between about 9.6 and about 9.8; about 9.2 and about 9.4 and about 3 and about 4 during each of said successive steps. At a pH between about 5 and about 6.5, an aluminate compound and an acid are simultaneously introduced into the slurry whereby alumina, in an amount of from 1.5 to 10 percent, is deposited onto the pigment. Finally, the pH of the slurry is adjusted to a value of about 7.5, the slurry filtered and the recovered pigment washed and dried. The essence of the process disclosed in this patent is stated to reside in close control of (1) the temperature and pH of the slurry to assure close contact between the pigment and the soluble silicate prior to initiation of precipitation of the silica and (2) the speed of precipitation (through a step-wise neutralization procedure) of the silica coating.
While the above continuous type processes purportedly overcome the major shortcoming of the previously known batch processes, i.e., excessively long processing times, adaptation of such continuous processes generally will entail significant capital investments in process equipment and controls to convert most current commercial production facilities to continuous operations. Thus, an improved batch-type process capable of producing titanium dioxide pigments having deposited thereon separate and discrete coatings of silica and alumina in shorter process times would represent an advancement in this art.